1. Field of the Invention
The present invention relates to an optical recording head, and more particularly to an optical recording head which has an optical valve element array made of PLZT, LC or the like or a self-emitting element array composed of LEDs and records an image (latent image) on a recording medium such as a photosensitive material and an electrophotographic photoreceptor.
2. Description of Prior Art
As a device for drawing an image (latent image) on a photographic paper or film made of a silver-halide sensitive material or an electrophotographic photoreceptor, an optical recording device which comprises a plurality of optical valve elements using PLZT arranged in a line and controls the optical valve elements to be on or off individually is generally known.
In this type of optical recording device, a two-dimensional image is drawn on a photosensitive material by projecting light, which passed through a plurality of optical valve elements arranged in a line (in a main-scanning direction), onto the photosensitive material and moving the photosensitive material in a sub-scanning direction. In the optical recording device, a great exposure is an indispensable condition for achieving high-speed drawing.
In order to satisfy the condition, the present inventor aimed at providing an integrator on the light emitting side of an optical fiber array for converting light from a light source into a line of light. The area of the light emitting end face of the integrator is made equal to the cross sectional area of the optical fiber array.
As for drawing of a color image using a color photographic paper, there is a known method in which lights of three primary colors are successively projected while switching colors by means of, for example, a color wheel located between a light source and an optical fiber array. The color wheel has R, G, B filters placed at equal intervals around its rotation axis, and is driven to rotate so that switching of the R, G, B filters is executed in synchronization with drawing of one line of each of the three primary colors. The diameter of such a color wheel is substantially proportional to the area of the light incident end face of the optical fiber array.
By the way, as the speed of drawing increases, the cycle of switching the R, G, B filters becomes shorter. This means that the rotation speed of the color wheel needs to be increased. In general, in order to rotate the wheel at a high speed, it is advantageous to decrease the diameter of the wheel. This means that the area of the light incident end face of the optical fiber array becomes smaller, and the illuminating area on the optical valve elements is in turn reduced, due to the above-mentioned proportional relationship. Consequently, higher accuracy is required in positioning the optical valve elements. However, the positioning accuracy requirement can be eased by setting the cross sectional area of the integrator to be greater than the area of the light emitting end face of the optical fiber array. In this case, however, the exposure on the photosensitive material is lowered, and the primary object, namely high-speed drawing, can not be achieved.
Meanwhile, in order to draw a color image at a high speed, Japanese Laid-Open Publication No. 6-347925 proposes a printer provided with three optical recording heads. In such a printer, the three optical recording heads are arranged in a sub-scanning direction, and a so-called tandem system in which an image is composed/recorded by exposing a recording medium (photographic paper) transported in a plane direction successively with an introduction of a predetermined time lag. When exposure is performed by a single head while successively switching the three primary colors B, G, R, the exposure of each color with respect to each pixel is inevitably reduced to one third because of switching. The structure using three heads does not cause such a reduction in exposure, thereby achieving high-speed drawing.
However, with the tandem system, it is difficult to accurately transport the photographic paper in a plane direction through a region where the three recording heads are arranged. More specifically, the exposure points of the B, G, R lights on the recording medium do not coincide with each other due to variations in transport speed of the photographic paper, a zigzag movement of the photographic paper or stretching of the photographic paper itself, resulting in color deviation or running. Moreover, in order to provide images of high quality with this kind of optical recording device, it is preferred to measure the quantity of light of each optical valve element and adjust the quantity of light according to the measured value. However, in the three-head structure of the tandem system, since three calibrators are required, the cost is increased.